Tanning with furfural



United States Patent TANNING WITH FURFURAL Bernard A. Roll, 8312A W. Center St., Milwaukee 10, Wis.

No Drawing. Filed Oct. 7, 1957, Ser. No. 688,434

Claims. (Cl. 8- 94.33)

This invention relates to a method of tanning collagen protein materials such as hides, skins, pelts and the like.

Tanning agents which convert hides or skins into leather have traditionally been applied in aqueous solutions. In recent years, systems have been developed for applying tanning agents from non-aqueous solutions wherein the solvent is usually a polar solvent, such as acetone or ethyl alcohol.

In the aqueous system, the hide or skin is initially prepared for tanning by subjecting itto a liming process wherein the hide is treated with an aqueous solution of lime or other alkaline materials and depilatory agents to remove the hair and swell the hide to thereby affect a greater separation of the individual protein fibers of which the hide is composed. The liming process influences the degree of suppleness and othercharacteristics of the tinished leather as Well as making the hide more receptive to the hating, pickling and tanning processes which are subsequently applied.

Following the liming process, the limed hide is treated with a solution of a de-liming salt, such as ammonium sulphate or ammonium chloride, in thepresence of a proteolytic enzyme. The de-liming salt is mildly acidic and neutralizes the lime and other alkaline materials and reduces the swelling incurred during the liming process, while the proteolytic enzyme acts to solubilize some of the unwanted constituents of the hide and thereby facilitates their removal by what is referred to as a hating action. V

The de-liming or hating process is followed by a pickling operation in which the de-limed skin is treated with an aqueous solution of an acid and salt. In this process, sulphuric, hydrochloric and formic acids are commonly used and the salt employed may take the form of any common salt, such as sodium chloride, sodium acetate or the like.

The particular pH value of the pickled hide is determined by the specific tanning agent to be subsequently used, and the acid in the pickling treatmentfunctions to control or maintain the pH of the hide at the desired value. If the hide is to be tanned with chromium sulphate, the pickling process will be such to condition the hide at a pH value of approximately 2.0 to 3.5, while if the skin is to be tanned with vegetable extracts the pH must be maintained at a value of 3.5 to 4.5. The salt functions to prevent swelling of the skin or hide under the acidic conditions.

After the pickling process, the pickled hide is subjected to the action of a tanning agent. In the aqueous system, the most commonly used tanning agents are chromium sulphate or chromium chloride and the so-called vegetable tanning materials derived from various woods, barks, nuts, etc. In both processes, the pH, temperature and concentrations of the tanning agents are extremely critical and must be accurately controlled.

After tanning, both the vegetable and chrome tanned leathers are treated with oils in a fatliquoring process. This process is essential to provide the leather with sulfiice cient strength and the right temper for its intended use. As the skin or hide is in a completely hydrated condition at this time, both the chrome and vegetable tanned stock are dried in a drying process. The drying of the stock is a very critical operation in determining the properties and characteristics of the leather.

In the solvent system of tanning, the tanning agent, such as chromium chloride or the common vegetable tanning agents, are applied from solutions of a polar solvent rather than water. The treatment of the stock prior to the application of the tanning agent is essentially the same as in the case of aqueous tanning. However, the characteristics of the leather obtained as well as the manner in which the tanning agent is absorbed by the stock are greatly different in the two systems. The solvent system of tanning has the advantages of the higher degree of tannage in a given period than the water system and the pH and concentration variables have comparatively little efiect. The solvent system of tanning produces leather having greater softness and pliability and excellent tensile and burst strength without the close control of pH, concentration of the tanning agent, etc. which are necessary in aqueous tanning.

Despite the inherent advantages of the solvent system of tanning over the aqueous system, the solvent systems have not attained commercial importance. vThe reason for this is that the costof the polar solvents makes it mandatory that they be completely removed from the stock after each tanning cycle. The problem of reclaiming the solvent absorbed by the stock is tremendously difiicult in practice since these solvents are extremely hazardous to handle. Costly installations are. required to permit the use of the solvent system in tanneries and the present equipment used for the water system cannot be converted readily to the solvent system. Q 7

The present invention relates to anovel method of tanning protein materials which differs from both the aqueous and solvent systems and offers important improve: ments not found in either system. According to the pres ent invention, furfural is employed as the tanning agent without the use of a solvent.

Generally, the process consists of immersing the pickled stock in theliquid furfural. The tanning action proceeds very rapidly even without milling action or agitation which is employed in conventional tannages. After the complete penetration of-the stock by the furfural, the stock is removed from the tanning solution and air dried to complete the tanning operation. v

The color of the leather tanned by the present process ranges from a very light tan to a dark brown and the grain surface is exceptionally smooth. .The strength characteristics compare to that of chrome tanned. leather and for many important applications, including shoe uppers or soling material,"the leather tanned by the present process requires no fatliquoring for its strength, temper and stretch characteristics are entirely suitable without the addition of oils.

The furfural employed as the tanning agent has the empirical formula C H O It is an amber colored liquid having a boiling point of about 161 C. The tannning agent consists solely of furfural without the addition of any other solvents or tanning materials. may be a commercial grade containing small amounts of impurities which do not affect the basic properties of the material. i

In the tanning process of the invention, the hide orskin is initially limed as in the case of a conventional tanning process. In this regard, the hide is treated with an aqueous solution of lime or other alkaline materials and a depilatory agent to remove the hair and swell the The furfural After the liming process, the limed hide may be treated by a hating or de-liming process in which the hide is neutralized with a slightly acidic material, such as an aqueous solution of ammonium-sulphate or ammonium chloride, in the presence of a proteolytic enzyme. The proteolytic enzyme removes certain muco-proteins which tend to act as cementing substances between the individual skinfibers. The hating process, improves the grain and tensile strength of the leather, its suppleness and feel, the receptiveness of the hide. to subsequent tanning processes and the appearance of. the grain surface to produce. a smooth, clean grain as opposed to the undesirable appearance of a high, mottled grain in the finished leather.

However, with the present tanning process, it is. possible .to eliminate thebating operation which is necessary in the conventional tanning processes. Inthe present process, the furfural absorbs small amounts of the degraded proteins from the. pickled stock which are generally removed from the stock in a hating operation.

After the liming process, the stock is pickled in an aqueous solution containing an acid and a salt. The acid may take the form of any conventional acid, such as hydrochloric, sulphuric, formic or the like, and the salt may be any common salt, such as sodium chloride, sodium acetate or the like. The acid functions to control or determine the pH of the stock and the salt serves to prevent swelling of the skin under the acidic conditions.

It is essential that the pickled hide have a pH less than 6.4 and generally in the range of 0.77 to 6.4 in order to be successfully tanned by immersion in furfural. The temper as well as the color of the leather are controlled by the degree of acidity or thepH value of the pickled stock. With a lower pH value in the above mentioned range, the leather is light colored and mellower than with a higher pH value in the above range. In addition, the salt should be present in the pickling solution in a 0.2 molal to a 2.0 molal concentration. A salt concentration of less than 0.2 molal results in a material that is black and brittle and has no leather properties, while, a salt concentration of over 2.0 molal will produce a. material having a leathery feel but hard and bony with a pebbled surface.

Example 1 300 grams of limed steer hide was pickled in a solution containing 1500 ml. of water, 90 grams of sodium chloride and 30 ml. of concentrated hydrochloric acid. The pickling solution and the stock reached an equilibrium pH of 0.77 in about twelve hours.

The pickle stock was thenimmersedinlSOO ml. of furfural. In twelve hours the stock was completely penetrated by the furfural and the stock was then removed and air dried at room temperature at approximately, 70 F. The resultant leather had a. very. mellow. temper and a very light tan color.

Example 2 300 grams of limed steer hide was pickled in 1500 m1. of water, 90 grams of sodium chloride, ml. of concentrated hydrochloric acid and 15 ml. of glacial acetic acid. The equilibrium pH of the stock and pickle solution was 1.8.

The pickled stock was immersed in 1500 ml. of' furfural for twelve hours, at the end ofwhich; period the furfural had completely penetrated the stock. The stock was air dried at room temperature and the resultant leather had a very mellow temper and a very light tan color.

Example 3 The procedure was the same as" that set'forth in Example 1 except that the pickle solution containedS grams of sodium acetate in addition to the 90 grams of sodium chloride and contained 15 ml. of concentratedhydrof "amen;

chloric acid and 15 ml. of glacial acetic acid. The equilibrium pH of'the stock and the pickle solution was 3.4.

After immersion in the furfural for twelve hours the finished leather had a mellow temper and a tan color.

Example 4 The procedure was similar to that set forth in Example 1 except that 61 grams of sodium chloride and 10 grams of sodium acetate were used in the. pickle solution along with 15 ml. of concentrated hydrochloric acid and 15 ml. of glacial acetic acid. The equilibrium pH of the stock and the pickle solution was 3.7.,

After immersion in the furfural for twelve hours the stock was removed and air dried. The finished leather had a firm temper and a light brown color.

Example 5 The procedure was the same as that set forth in Example l except that 26 grams of sodium chloride and 37 grams of sodium acetate were used along with 12 ml. of concentrated hydrochloric acid and 13 ml. of glacial acetic acid. The equilibrium pH of the stock and the pickle solution was 4.4.

After immersion in the furfural the leather rated a firm temper and a brown color.

Example 6 The stock was treated similar to that of Example 1 except that the pickle solution contained 6 ml. of concentrated hydrochloric acid and grams of sodium chloride. The equilibrium pH in this case was 4.0. The leather had a firm temper and a brown color.

Example 7 The procedure was the same as Example 1 except that 7 ml. of concentrated hydrochloric acid was employed along with 90 grams of sodium chloride. The equilibrium pH was 3.5. The leather had a mellow temper and a tan color.

Example 8 The precedure was similar to that of Example 1 except that 5 ml. of concentrated hydrochloric acid and 90 grams of sodium chloride were used in the pickle solution. The equilibrium pH was 4.7 and the leather was fairly firm and had a brown color.

Example 9 The procedure was the same as in Example 1 except that the pickle solution consisted of 4 ml. of concentrated hydrochloric acid and 90 grams of sodium chloride. The equilibrium pH was 6.5. The finished leather was very firm and had a medium brown color.

In Examples 1 through 9 all of the leathers werecharacterized as having a good leathery feel. The shrinkage temperatures of these leathers varied from about F. to F.

Example 10 The procedure was the same as Example 1 except that 17 grams of sodium chloride were employed with 12 ml. of concentrated hydrochloric acid and 13 ml. of glacial acetic acid-in;the pickle solution. The equilibrium-pH was/1.6.

In this case the leather had a very firm temper and a medium brown color.

Example 12 The procedure was the same as Example 1 except that 9 grams of sodium chloride were employed with 12 ml. of concentrated hydrochloric acid and 13 ml. of glacial acetic acid in the pickle solution. The equilibrium pH was 4.3. In this case the finished stock was hard and could not be classified as leather.

The Examples 10 through 12 illustrate the importance of the salt concentration in the pickle solution when subsequently tanning with furfural. The Example 12 indicates that 9 grams of sodium chloride in a 1500 ml. of water in the pickle solution is definitely insufficient to provide a pickled stock which can be tanned with furfural. Example 11 is a borderline case in that the finished stock has somewhat of a leathery feel, but is too firm for practical purposes, While Example 10 shows that 26 grams of sodium chloride per 1500 ml. of water is a definitely sufficient salt concentration for the pickle stock when subsequently tanned with furfural.

Example 13 The same procedure was employed as in Example 9 7 except that 3 ml. of concentrated hydrochloric acid were Example 14 p The same procedure was employed as in Example 13 except that 180 grams of sodium chloride were used in the pickle solution instead of 90 grams. The equilibrium pH in this case was 7.10.

The resulting stock could be classified as leather, as it had a leathery feel, did not crack on flexing, and could have use for certain purposes. However, the temper was very firm and the color was a medium brown.

The grain surface of this leather was very unattractive, having a pebbled look. while the stock produced by this example could be classified as leather it would be a very inferior grade of leather.

Example 15 300 grams of limed stock was pickled in a solution containing 1500 ml. of water, 90 grams of sodium chloride, and 15 ml. of concentrated hydrochloric acid. The stock and pickle solution reached an equilibrium pH of 1.35.

The pickle solution and the stock were gradually neutralized by adding sodium hydroxide over a period of several days. The new'equilibrium pH thus obtained was 7.31.

Thispickle stock was then immersed in 1500 ml. of furfural and the furfural completely penetrated the stock in about 12 hours at which point the stock was air dried at room temperature.

The resultant stock could not be considered acceptabl: as leather having approximately the same properties and characteristics as the stock obtained in Exampic 13.

The present process of tanning using furfura-l as the tanning agent is entirely different in function and results from either the conventional aqueous or solvent type tanning processes. There have been attempts in the past to employ furfural in combination with other 7 The tensile strength of the leathers produced by the present method of tanning are consistently higher than those tanned in aqueous systems. A typical shoe upper leather, chrometanned, vegetable retanned, fatliquored and dried in the usual manner consistently'shows a tensile strength of 15% below a similar leather tanned by the present method. Furthermore, the leather tanned by the present method in the comparative test had not pickled stock that is necessary in the conventional tanning processes. As the pH of the furfural can be varied without any noticeable affect on the tanned leather, this means that the furfural can be employed to tan various types of leather, while in the former processes it was necessary to have a different tanning bath for each type of leather due to the fact that the pH of the tanning agent could'not be varied to any great degree.

It has been found that in a commercial operation the most successful tanning and uniformly reproduceable results are obtained when the furfural is repurified by distillation after having been used about four times. During use, less than 10% of the furfural is absorbed by the stock and the remaining of the furfural can then be used to tan the next batch of pickled stock, and this process can be repeated for about four cycles without any difiiculty. After about the fourth cycle, however, the tanning time is increased due to the fact that the furfural absorbs small amounts of moisture, salt and degraded protein from the pickled stock. When the furfural has reached this point of contamination, it can be easily separated from the impurities by distillation.

The process of tanning with furfural provides a finished leather having an excellent tensile strength and grain structure and eliminates the bating operation which is necessary with the conventional tanning methods. Furthermore the fatliquoring operation in which fat is applied to the tanned stock can also be eliminated with the present process.

The furfural being employed as the tanning agent does not require a solvent, such as water or a polar solvent. This considerably simplifies the process and eliminates the problem of solvent recovery as is the case in the solvent system of tanning. The furfural tanning agent becomes permanently afiixed to the hide or stock to the extent that it cannot be separated by water, acids or common solvents.

With the use of the furfural, the temper and color of the leather are easily controlled by simple adjustments of the pickling process proceeding the application of the furfural and no adjustment of the pH of the furfural is necessary to change the color and temper of the leather.

The furfural tanning process can be used with pickled hide having a wide range of pH, as compared with the narrow pH ranges that are necessary with the conventional tanning processes, and secondly, no pH control of the furfural itself is required.

Various modes of carrying out the invention are con templated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. A process of tanning collagen protein material, comprising subjecting'the protein material having a pH in the range of 0.77 to 6.40 to the action of a tanning agent consisting solely of furfural and being substantially free of water and other tanning agents.

2. A process of tanning hides, comprising pickling the '2'. hide in an acidic medium toprovidethehidewithia pH value up to 6.4, and thereafter subjecting the:,pickled hide to the action of a tanning agent consisting solely of furfural containing minorimpuritiesnot effecting-mechanacteristics of the same.

3. A process for tanning hides, comprising liming the hides, subjecting the hides to. the action of a; pickling solution containing a dissociable metal salt; and an acid to provide the hides with a pH in the range of 0.77 to 6.40, said salt being present in said picklingsolution in a 0.2 to 2.0 molal concentration, and thereafter applying a tanning agent to the pickled hide consisting ofsubstam tially 100% furfural.

4. A process for tanning collagen proteinmaterial, comprising liming the material, pickling the material di rectly after the liming operation to providethe material with a pH in the range of 0.77 to.6.4, and thereafter immersing the pickled material in a tanning agentxzonsistingof substantially 100% furfural.

8 '5". A process for tanning collagen protein material,

comprising liming the material, pickling thematerial in a,

References Cited in the file of this patent Gustavson: The Chem. of Tanning Processes, Academic Press Inc., N.Y., 1956, pp. 246 and 247.

Gourlay: Rev. Tech. des Ind. du Cuir, December1956, XLVIII, No. 12, pp. 263-265.

Brother et al.: Protein Plastics from Soybean Products, Ind. Eng. Chem., 30:1236-40 1938. 

3. A PROCESS FOR TANNING HIDES, COMPRISING LIMING THE HIDES, SUBJECTING THE HIDES TO THE ACTION OF A PICKLING SOLUTION CONTAINING A DISSOCIABLE METAL SALT AND AN ACID TO PROVIDE THE HIDES WITH A PH IN THE RANGE OF 0.77 TO 6.40, SAID SALT BEING PRESENT IN SAID PICKLING SOLUTION IN A 0.2 TO 2.0 MOLAL CONCENTRATION, AND THEREAFTER APPLYING A TANNING AGENT TO THE PICKLED HIDE CONSISTING OF SUBSTANTIALLY 100% FURTURAL. 